Hot-Pressed Transparent Ceramics and Ceramic Lasers

ABSTRACT

A transparent polycrystalline ceramic having scattering and absorption loss less than 0.2/cm over a region comprising more than 95% of the originally densified shape and a process for fabricating the same by hot pressing. The ceramic can be any suitable ceramic such as yttria (Y 2 O 3 ) or scandia (Sc 2 O 3 ) and can have a doping level of from 0 to 20% and a grain size of greater than 30 μm, although the grains can also be smaller than 30 μm. Ceramic nanoparticles can be coated with a sintering aid to minimize direct contact of adjacent ceramic powder particles and then baked at high temperatures to remove impurities from the coated particles. The thus-coated particles can then be densified by hot pressing into the final ceramic product. The invention further provides a transparent polycrystalline ceramic solid-state laser material and a laser using the hot pressed polycrystalline ceramic.

CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.12/620,613 filed on Nov. 18, 2009, which in turn claims the benefit ofpriority based on U.S. Provisional Patent Application No. 61/138,730filed on Dec. 18, 2008, both of which are hereby incorporated byreference into the present application in their entirety.

TECHNICAL FIELD

The present invention relates to hot-pressed transparent Yb:Y₂O₃ andother advanced ceramics and processes of making the same. The presentinvention also relates to a ceramic laser in which the gain medium is ahot pressed ceramic rod, disc or slab.

BACKGROUND

Advanced ceramics such as ytterbium-doped yttria (Yb:Y₂O₃) are excellentlaser media due to their hardness, strength, and transparency in therange of 0.4 to 10 μm. Their thermal properties enable the laser tooperate at higher temperatures and dissipate heat generated during laseroperation better than other laser materials, such as yttrium aluminumgarnet (YAG).

However, single crystal Yb:Y₂O₃ is difficult to produce in the largesizes and necessary configurations for high-power lasers.Polycrystalline Yb:Y₂O₃ can be used in such high-performanceapplications if it is produced in a manner as to produce a fine grainedtransparent material with clean grain boundaries, very low porosity, andless than 10 ppm levels of impurities.

Transparent polycrystalline Yb:Y₂O₃ laser material conventionally hasbeen produced by sintering ceramic powders in a process whereby ananosized ceramic powder is cold-pressed into a green body having thedesired shape which is then heated without pressure to form the finalproduct. This process is different from sintering by hot-pressing, inwhich the ceramic powder is heated under pressure to form the finalproduct. Although the powder being hot-pressed must not melt to a greatextent, some melting of secondary phases in the powder or surfacemelting can be allowed, and in the case of porcelains and clay products,the melting of these secondary phases can act as an intrinsic sinteringaid to “glue” the primary solid particles together with a glassy phaseto form the solid ceramic material.

However, advanced ceramics do not have these intrinsic sintering aidsand therefore some extrinsic sintering aid must be added. For example,materials such as Yb:Y₂O₃ often are mixed with a secondary material suchas lithium fluoride (LiF) as a sintering aid. Some sintering aids mayliquefy at or somewhat below the primary material's densificationtemperature thereby promoting liquid phase sintering. Other sinteringaid materials exhibit higher solid-state diffusion coefficients than theprimary material's self-diffusion coefficient. The secondary materialmay conversely have a lower solid-state diffusion coefficient thatprevents exaggerated grain growth and promotes grain boundary refinementand pinning. The sintering aid may also simply clean or etch the primarymaterial's surfaces thereby enhancing solid-state diffusion.

For small batches, the powdered sintering aids can be mixed with thepowder to be sintered with a mortar and pestle. In larger batches,mixing can be accomplished by ball milling, attritor milling, high shearwet milling, and variations or combinations of these methods. However,in the case of optical or laser quality polycrystalline materials,homogeneity must be measured on the nanometer scale, and mechanicalmixing results in homogeneity of the sintering aid that is only in thehundreds of microns range, a level of homogeneity that is several ordersof magnitude too high to produce optical or laser qualitypolycrystalline material. Inhomogeneity of the sintering aid within theceramic powder results in areas that have too much sintering aid andother areas that have little or no sintering aid. While this isgenerally not too important in fabrication of materials that arerelatively easy to sinter or in opaque materials, it is a major problemin the fabrication of transparent ceramics.

The sintering aid must also be removed from the densified material toprevent it from being trapped and forming pores in the material. In thecase of optical ceramics, if the sintering aid is not effectivelyremoved, the pores formed by the trapped material can cause highscattering and absorption losses in the final article. The scatteringsites in such ceramic materials are typified by inclusions and voidsthat appear white when viewed in reflected light. The absorbing regionsare dark when viewed in both transmitted and reflected light. In such acase, the article does not possess uniform optical losses, andconsequently the yield is poor, costs are high and large size anddifferent shapes are not possible to manufacture.

In the case of Y₂O₃ ceramics using LiF as a sintering aid, theinclusions contributing to optical scattering are due to trapped LiFthat was not removed during sintering and by compounds that resultedfrom impurities in the starting ceramic powder and sintering aid.Additional scattering is caused by the presence of voids, i.e., pores,that possess very high scattering efficiencies. The absorption lossesare caused by oxygen vacancies which arise from the presence of carbonrelated impurities. The carbon-related impurities react with oxygen inthe Y₂O₃ to produce CO/CO₂ volatile gases and leave behind vacancies inthe molecular structure, causing the material to turn black. The extentof the absorbing regions is influenced by the LiF sintering aid, whichreacts with the carbon to form volatile carbon-fluorine species that canbe easily removed. Not enough LiF leads to oxygen vacancies, whereasexcess LiF gets trapped and leads to optical scattering.

Transmission losses in such transparent ceramics are also due toabsorption caused by impurities including transition metals and otherionic species present in the ceramic. Scattering in transparent ceramicscan be minimized by optimizing the processing conditions such as powdersize distribution control, sintering pressure, and sinteringtemperature. However, absorption loss caused by the presence of varioustransition metals and other ionic impurities in the ceramic is criticalsince such defects lower the transmittance and have a detrimental effecton lasing.

Sintered Yb doped Y₂O₃ ceramic materials have had a demonstrated use aslaser materials. See A. A. Kaminskii et al. “Lasing and RefractiveIndices of Cubic Yttrium Oxide Y₂O₃ Doped with Nd³⁺ and Yb³⁺ Ions.”Crystallography Reports, Vol. 48, No. 6 (2003) pp 1041-1043; and J. Konget al. “9.2W diode-end-pumped Yb:Y₂O₃ ceramic laser.” Applied PhysicsLetters, Vol. 86 (2005) pp. 161116-1-161116-3. Sintering is achievedusing nano-sized starting powder that is uniaxially cold pressed andthen cold isostatically pressed into a green body, which is then heatedin vacuum without external pressure or load. The nanopowder is a strictrequirement and highlighted by previous authors in their patents. SeeU.S. Pat. No. 6,825,144 to Hideki (polycrystalline laser gain media arelimited to crystals having a mean grain size of less than 20 μm, andlaser quality ceramic cannot be made if the grains are larger).

Nano-sized powder has a huge driving force to lower the surface energyand so heating at elevated temperatures allows densification (sintering)to take place whereby the grains grow slightly and the surface energy islowered. Typically, the grains are a few microns in size. However, thegrain size must remain small so that pores can be removed to enableproduction of laser quality ceramics. If prolonged times or highertemperatures are used in an attempt to eliminate porosity, the grainsgrow larger than about 25 μm, and it becomes difficult to remove thepores. In addition, in such cases, significant grain growth will occur,which makes it even more difficult to remove the pores.

It has been assumed that it is not possible to make a laser qualityceramic from Yb doped Y₂O₃ using hot pressing. In hot pressing, asintering aid is mixed with the powder. The mixed powder is placed in ahot press, evacuated, and then heated up to the densificationtemperature, at which time a load of several thousand psi is applied.The problem has been that the sintering aid, which prevents carboncontamination from the hot press die, typically leads to grain growth.In fact grains are typically larger than 30 μm and more typically largerthan 100 μm. Therefore it was widely believed that hot pressed Yb dopedY₂O₃ will not produce laser quality ceramic since the porosity cannot bereduced. In fact this rationale has led the other research groups awayfrom using hot pressing.

If one could solve the problem of scattering using hot pressing, thennot only could laser quality ceramics be attainable, but otheradvantages of hot pressing could be exploited. These include scalabilityto large sizes and complex shapes, relatively short processing times,and mass production. Manufacturing costs are lowered and the potentialof making high power lasers, including >>KW class lasers, becomes moreviable.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

The present invention provides a transparent polycrystalline ceramichaving combined scattering and absorption losses of less than 0.2/cmover a region comprising more than 95% of the originally densified shapeand further provides a process for fabricating the same by hot pressing.The ceramic can be any suitable ceramic such as yttria (Y₂O₃) or scandia(Sc₂O₃) and can have a doping level of from 0 to 20% and a grain size ofgreater than 30 μm, although smaller grains are also possible. In aprocess for making a transparent polycrystalline ceramic in accordancewith the present invention, ceramic nanoparticles can be coated with asintering aid so that adjacent Y₂O₃ (or Sc₂O₃) powder particles are notin direct contact with each other and then baked at high temperatures toremove impurities from the coated particles. The thus-coated particlescan then be densified by hot pressing into the final ceramic product.The present invention further provides a laser based upon ceramicYb:Y₂O₃ (or Sc₂O₃) fabricated by hot pressing which has grains largerthan 30 μm and has high optical quality suitable for laser emission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are SEM photographs of Y₂O₃ ceramic powder (FIG. 1A) andLiF powder (FIG. 1B).

FIGS. 2A and 2B are SEM photographs of mixed Y₂O₃ and LiF powders. FIG.2A depicts a sample of Y₂O₃ and LiF powders that have been mechanicallymixed as in the prior art; FIG. 2B depicts a sample wherein LiF has beendissolved and precipitated on Y₂O₃ in accordance with the presentinvention.

FIG. 3 depicts aspects of a process for making a transparent Yb:Y₂O₃ceramic in accordance with the present invention.

FIGS. 4A and 4B are plots of optical loss as a function of wavelengthfor a conventional transparent ceramic medium (FIG. 4A) and atransparent ceramic medium in accordance with the present invention(FIG. 4B).

FIGS. 5A and 5B are micrographs illustrating the porosity of atransparent Y₂O₃ ceramic prepared in accordance with conventionalprocesses (FIG. 5A) and in accordance with the present invention (FIG.5B).

FIG. 6 depicts an exemplary configuration of a laser using a transparentYb:Y₂O₃ ceramic in accordance with the present invention.

FIGS. 7A and 7B are plots showing the performance of a laser using atransparent Yb:Y₂O₃ ceramic in accordance with the present invention interms of output power vs. absorbed pump power (FIG. 7A) and output pulseenergy vs. absorbed pulse energy (FIG. 7B).

DETAILED DESCRIPTION

The aspects and features of the present invention summarized above canbe embodied in various forms. The following description shows, by way ofillustration, combinations and configurations in which the aspects andfeatures can be put into practice. It is understood that the describedaspects, features, and/or embodiments are merely examples, and that oneskilled in the art may utilize other aspects, features, and/orembodiments or make structural and functional modifications withoutdeparting from the scope of the present disclosure.

For example, although the product and process in accordance with thepresent invention may be described in terms of an exemplary embodimentin which a doped yttria ceramic is made from Yb³⁺ doped Y₂O₃ (Yb:Y₂O₃)ceramic nanopowders using LiF as a sintering aid, one skilled in the artwould understand that in other embodiments, any suitable ceramicpowders, including undoped Y₂O₃ ceramic powders and doped or undopedscandia (Sc₂O₃) powders, and any suitable sintering aid can be used asappropriate to make a transparent ceramic having desired optical orother properties. For example, undoped transparent ceramics can be usedin applications such as windows or as lens covers. On the other hand,due to the absorption characteristics of certain elements, the dopantand doping level in a doped transparent ceramic can be chosen so thatthe final material exhibits the desired transmission and/or absorptioncharacteristics for the application in which it is to be used.

In an exemplary embodiment in accordance with the present invention, ahigh-quality transparent ytterbium-doped yttrium oxide (Yb:Y₂O₃) ceramicmaterial can be made by coating a high-uniformity loose Yb:Y₂O₃ ceramicnanopowder with a dissolved solution of a sintering aid such as LiF,drying the coated powder by baking at high temperatures in air, and thendensifying the dried powder by hot-pressing to form the final ceramicmaterial. A transparent Yb:Y₂O₃ ceramic material produced in accordancewith this process exhibits scattering and absorption losses of less than0.2/cm over a region comprising more than 95% of the original densifiedshape. The ceramic can have a doping concentration of from 0 to 20%, andcan have both smaller (<30 μm) and larger (>30 μm) grains with a mono-,biomodal, and/or multimodal dispersion of the grains throughout thematerial, while still exhibiting low attenuation, low scattering, andgood suitability for lasing.

A transparent polycrystalline Yb:Y₂O₃ ceramic in accordance with thepresent invention can be made using a high-purity uniformly nano-sizedYb:Y₂O₃ powder having no hard agglomerates such as that described in theconcurrently-filed patent application entitled “Yb:Y₂O₃ CeramicPowders,” (Navy Case No. 98,968), which shares inventors in common withthe present invention and is hereby incorporated by reference into thepresent application.

As noted above, transparent Yb:Y₂O₃ ceramics can be made by densifyingthe yttria ceramic powder. Traditional processes used for densifyingyttria attempted to either not use any sintering aid and to rely on thesintering of nanoparticles with very fine particle size distribution, orto use relatively large amounts (>1% by weight) of sintering aid toovercome the possibility of having regions with no sintering aid.However, the traditional process for achieving a fine nanoparticle sizedistribution results in the use of only ˜20% of the prepared powder. Theother 80% of the powder is too large or otherwise unsuitable forsintering without use of a sintering aid. Densification with a sinteringaid allows the use of greater than 90% of the powder produced, but itcomes at the cost of more involved heating/pressure schedules, highertemperatures resulting in larger grain growth, and a final product yieldof ˜50% because, as described below, the final product has too muchabsorption and scattering to be useful.

In conventional processes using a sintering aid, the sintering aid, forexample, LiF powder, is mixed into the ceramic powder using a mortar andpestle, or by ball milling, attritor milling, high shear wet milling,and various combinations of these processes. However, as can be seen inFIGS. 1A and 1B, the LiF powder shown in FIG. 1B is of a much largerscale than the yttria powder shown in FIG. 1A, and this difference inparticle size makes the mixture of yttria and LiF far from homogeneous.This inhomogeneity is seen in the mechanically mixed yttria/LiF powdershown in FIG. 2A, which clearly illustrates the presence of discrete LiFparticles within the yttria powder.

As discussed above, the presence of these discrete particles within thefinal ceramic product can create scattering and attenuation losses,making the final product unsuitable as a high-quality solid state lasermaterial.

The scattering sites are best reduced or eliminated by homogeneouslydistributing the sintering aid as a coating on the starting ceramicpowder particles so as to minimize direct contact of adjacent Y₂O₃particles (or Sc₂O₃ particles, or their doped particles). This allows areduction in the total amount of sintering aid used and consequentlyreduces the amount of unwanted reaction byproducts that are left in thematerial as scattering sites.

Thus, in accordance with the present invention, a high-qualitytransparent Yb:Y₂O₃ ceramic material can be made by coating the looseceramic powder with a dissolved solution of a sintering aid such as LiF,drying the coated powder by baking at high temperatures in air, and thendensifying the dried powder by hot-pressing to form the final ceramicmaterial. By preparing the ceramic powder in accordance with thismethod, more than 90% of the powder can be used. A ceramic materialprepared in accordance with this process can exhibit scattering andabsorption losses of less than 0.2/cm over a region comprising more than95% of the originally densified shape. In order to minimize thescattering and absorption losses and improve performance of the finalproduct, the powders can preferably be a uniform nano-scale ceramicpowder having no hard agglomerates such as the powder described in theconcurrently-filed patent application entitled “Yb:Y₂O₃ Ceramic Powders”described above.

In one exemplary embodiment of the process according to the presentinvention, nano-scale homogeneity of the sintering aid on suchnanopowders can be achieved by dissolving the LiF sintering aid in asuitable solvent, such as water and suspending the Yb:Y₂O₃ powder in thesolution. In such a case, the LiF sintering aid is not actually incontact with the particles since it is dissolved into Li⁺ and F⁻ ions.The ions are brought out of solution and into intimate contact with theYb:Y₂O₃ particles by drying or inducing precipitation. Care must betaken during this step to ensure that the LiF precipitates on theparticle surface and not as separate particles in the liquid. This canbe done by slowly creating a supersaturated solution of Li⁺ and F⁻ ionsthat heterogeneously nucleate on a surface (in this case the Yb:Y₂O₃powder) instead of nucleating in the liquid.

In another embodiment of the process according to the present invention,the LiF coating can be applied to the loose powders by spray coating.

Irrespective of the method of application, in accordance with thepresent invention, the LiF-coated loose powder can then be baked at hightemperatures for several hours (e.g., 600° C. in air for at least 6-12hours) to dry the powder. Baking the loose powder can also removevolatile species such as adsorbed hydrocarbons and other processinginduced impurities from the particle surfaces, thus improving the purityof the loose powder and reducing the potential for the creation ofscattering sites in the final product. The baking temperature should becarefully selected since baking at higher temperatures causesexaggerated grain growth in the subsequently densified body, while lowertemperatures do not completely remove the carbon species.

FIG. 2B illustrates a LiF-coated Yb:Y₂O₃ ceramic powder in accordancewith the present invention, in which the LiF sintering aid has beendissolved and precipitated onto the powder particles. As can be seen,particularly when compared with the powder shown in FIG. 2A in which theLiF sintering aid has been mechanically mixed, a ceramic powder having adissolved and precipitated sintering aid has far greater uniformity.This can produce a transparent ceramic material having less scatteringand attenuation loss, making it more suitable for use as a solid statelaser material, i.e., a ceramic laser material capable of demonstratinglasing.

As noted above, when the powder is coated with the LiF sintering aid, asthis invention teaches, greater than 90% of the powder can be used andthe final ceramic product yield is greater than 95%. This is due to acombination of factors.

In accordance with the present invention, the LiF concentration isreduced to only 0.2% because the homogeneity of the sintering aiddistribution is on the nano scale and there is no need to add largeramounts of sintering aid in an attempt to have no deficient regions. Inaddition, there is no need to hold the temperature of the hot pressaround the melting point of the sintering aid for a period of time in anattempt to increase homogeneity or to further hold the temperature ofthe hot press at the point where the sintering aid is removed becausethe distribution of the sintering aid is already homogeneous within theceramic powder on a nano scale, and in any case there is very littlesintering aid to be removed by evaporation and/or sublimation.

The even distribution of sintering aid achieved in accordance with thepresent invention also allows densification under less harsh conditionsthan traditionally used and allows critically needed flexibility in thehot pressing schedule.

Traditional hot pressing conditions tend to accelerate the formation andtrapping of reaction byproducts due to the use of higher temperaturesand pressures, necessary holds in the hot pressing schedule to attemptto remove by-products, and longer pressing times at elevatedtemperatures. In contrast, even a reduction in densification temperatureof 25° C. leads to a decrease in the amount of scattering sites andlight absorption regions. Similarly, a 25° C. change in the pressureapplication temperature can mean the difference between a clear and ahazy article.

In a process for making a transparent ceramic in accordance with thepresent invention, the heating rates can be reduced since less LiF meansless reaction with the yttria and thus fewer vacancies. The slowerheating rates allow a more precise control over the densificationdynamics of the yttria and a narrower temperature range over which thepressure is applied. For example, if it takes 5 minutes to applypressure, a 5° C./min heating rate results in a temperature increase of25° C., whereas a 20° C./min heating rate results in a 100° C.temperature increase. The slower heating rate also removes the need toadd another hold to allow application of pressure at a controlledtemperature. The holds lengthen the time where sintering aid/yttriainteractions occur, and they also allow neck formation that results intrapped pores. Thus, by reducing or eliminating the need for temperatureholds, the process of the present invention can significantly reduce thenumber of scattering sites in the final product.

In addition, although as described above the present invention canpreferably use a highly uniform ceramic nanopowder having no hardagglomerates, because the sintering aid is distributed on each particle,the process of the present invention can be used with non-optimizedpowders and can result in high product yields, and so can improve thecost-effectiveness of producing and using transparent advanced ceramicssuch as transparent Yb:Y₂O₃.

In addition, the ceramic laser materials described herein, such asytterbium doped yttria (Yb:Y₂O₃) or scandia (Yb:Sc₂O₃) ceramic lasermaterials which have been fabricated by hot pressing and whose grainsize is larger than 30 μm (but also could be smaller than 30 μm), can befabricated into at least three typical laser geometries, i.e., rod,disc, and slab, and lasers using these materials can be constructed. Toconstruct such a laser, the laser material is placed in a cavity whichcan be formed by external resonators or coatings deposited on thesurface of the material. The material is pumped by a diode laser orother suitable source at typically around 915, 940, or 976 nm, and laseremission occurs typically around 1080 nm (1.08 μm). In addition, as seenin FIG. 4B, such a laser can have a transmission loss of about 0.2/cm ina wavelength range of about 0.6 to about 6.0 μm.

EXAMPLES

These aspects of the present invention can be further understood in thecontext of the following examples.

Example 1

This example teaches the process for making a Y₂O₃ ceramic using asintering aid in accordance with conventional processes. In accordancewith conventional processes, LiF and undoped Y₂O₃ particles aremechanically mixed and then densified by hot pressing under thefollowing schedule: increase the temperature at 20° C./min to 950° C.and hold for 30 min.; increase the temperature at 20° C./min a secondtime to 1200° C. and hold for 30 min; and increase the temperature atramp 20° C./min a third time to 1650° C. and hold 2-6 hours under vacuumand at a pressure of 2000 to 8000 psi. The holds are necessary toattempt to evenly distribute the LiF after it melts at 850° C. and toallow extra time for the removal of the LiF.

Example 2

This example uses the same mixing and pressing conditions as describedin Example 1 above, but uses yttria doped with 10% ytterbium.

Example 3

This example teaches the process for making a transparent Y₂O₃ ceramicin accordance with the present invention. As described above, ahigh-quality transparent ceramic can be made by coating the ceramicpowder with a sintering aid, so that direct contact of adjacent Y₂O₃powder particles is minimized, and heat treating the coated powder priorto densification. Thus, in accordance with the present invention, inthis example, Y₂O₃ ceramic nanopowders can be coated with LiF as asintering aid and the coated powders heat treated at 600° C. for 4 to 18hours in air prior to densification.

FIG. 3 illustrates an exemplary embodiment of this process. Inaccordance with the present invention, coating the Y₂O₃ ceramicparticles with the LiF sintering aid can be achieved by dissolving LiFin water, adding the Y₂O₃ powder to the solution, and spraying themixture using sprayer 401 to form droplets 402 that travel into acontrolled heat gradient created by heating elements 403. Subjecting thecoated particles to heat, e.g., heat at 600° C. for 4 to 18 hours asdescribed above, supersaturates the Li⁺ and F⁻ ions, causing the ions tonucleate on the Y₂O₃ particles 404 as sintering aid LiF 405 and thus thecoated ceramic particles.

The coated ceramic particles can then be densified in hot press die 406having heating elements 407 and 3-ton load 408 shown in FIG. 3 using thesame hot pressing schedule as described above with respect to Example 1to produce transparent ceramic 409. The material can be made into anysuitable shape using hot pressing, such as the rod, disk, or slab shapesoften used in laser applications.

Transparent ceramic 409 has a high and uniform transparency over 95% ofthe total area. This permits higher yield and lower cost as well aspotential for large size fabrication.

In addition, as shown in FIG. 4B, the loss is consistently low over thematerial surface.

Example 4

Taking advantage of the flexibility in hot pressing parameters offeredby the coated samples, the powder is processed as described above inExample 3, but the hot pressing conditions can be changed to produce astronger finer grained sample with better optical and lasing qualities.In this example, the powder is densified by hot pressing with thetemperature being increased at a ramp rate of 20° C./min to 1000° C. andthen at a rate of 5° C./min to 1500° C., which is then held for 2 hoursat a pressure of 2000 to 8000 psi. Note that in this Example, there isno intermediary hold between the first and second temperature ramps.

Fabricating a Y₂O₃ ceramic with this heating/pressing schedule using amechanically mixed LiF sintering aid results in a barely translucentsample possessing gray and white splotchy regions, which is in starkcontrast to the transparent sample 409 shown in FIG. 3.

The lack of transparency and uniformity makes such a ceramic clearlyunsuitable as a high-quality solid state laser material. FIG. 4A is aplot showing the transmission loss for such a ceramic, and shows a lossgreater than 0.2/cm. In addition, the loss varies with position on theceramic surface and is random, limiting usefulness of ceramic material.

In addition, fabricating a ceramic material using particles coated withthe sintering aid in accordance with the present invention cansignificantly decrease the porosity of the final ceramic. FIG. 5A showsa micrograph of the internal porosity of a ceramic prepared inaccordance with Example 2 described above. As can be easily seen, thissample is highly porous, which can cause it to exhibit significantscattering loss and reduced strength.

In contrast, FIG. 5B shows a micrograph of the porosity of a ceramicmaterial prepared from LiF-coated ceramic nanoparticles in accordancewith Example 4 described above. As can be seen, this ceramic isessentially pore-free, and thus has significantly lower scatteringlosses and higher strength.

Thus, in accordance with the present invention, a high-quality,low-scattering, low-absorption transparent ceramic can be manufacturedwhich can be used in high power laser systems and as highly transmissivewindows and domes.

For example, a transparent ceramic material used as a solid state lasergain material (e.g., slab, rod, or disk) can be placed in a cavityformed by external resonators or coatings placed on the material'ssurface, and the material pumped by a diode laser or other suitablesource, typically at around 915, 940, or 976 nm, with laser emissiontypically occurring at around 1080 nm.

Example 5

An exemplary embodiment of a laser using a transparent ceramic lasermaterial in accordance with the present invention is shown in FIG. 6. Inthe exemplary embodiment shown in FIG. 6, a hot pressed 2% Yb:Y₂O₃transparent ceramic material 605, with grains larger than 30 μm, havingdimensions ˜2 mm thick by ˜3 mm diameter was placed in a laser cavityformed by a thin film deposited on one surface of the laser material 605and an external mirror 604. The thin film is high reflectivity >99.9% at1080 nm and high transmission >98% at the pump wavelength of 937 nm. Theexternal mirror 604 is a 50 cm concave mirror with 2% transmission atthe laser wavelength of 1080 nm. The other surface of the hot pressedYb:Y₂O₃ laser material is coated with an anti-reflection coating at 1080nm. The Yb:Y₂O₃ laser material is endpumped by pump diode 601 and pumpdelivery fiber 602 through lenses 603 and through the first filmcoating. Laser emission occurred through output coupler 606 at 1076 nm.

FIGS. 7A and 7B are plots showing the performance of this laser. As seenin FIG. 7A, a laser using a hot pressed 2% Yb:Y₂O₃ transparent ceramicmaterial in accordance with the present invention exhibits a 45% slopeefficiency of output power versus absorbed pump power This is derivedfrom the slope of FIG. 7A. Similarly, as can be seen from the plot inFIG. 7B, such a laser exhibits a 46% slope efficiency of laser pulseenergy output versus absorbed pulse energy.

Thus, a laser using a hot pressed 2% Yb:Y₂O₃ transparent ceramicmaterial in accordance with the present invention also exhibits lasingwhen the grain size is above 30 μm.

As noted above, a transparent ceramic material produced in accordancewith the present invention can have many other uses as well. Forexample, an undoped transparent ceramic such as an undoped Y₂O₃ (orSc₂O₃) ceramic can be used in many applications where a highlyshatter-resistant transparent material would be desirable, such aswindows, lens covers, etc. Alternatively, if it is desirable thatcertain wavelengths be attenuated, the base material and dopant can bechosen to achieve the desired attenuation/transmission properties.

In addition, coating the individual ceramic particles with the sinteringaid in accordance with the present invention allows for greaterflexibility in the densification process, making mass fabrication of anyof these transparent ceramics less difficult and less expensive.

Although particular embodiments, aspects, and features have beendescribed and illustrated, it should be noted that the inventiondescribed herein is not limited to only those embodiments, aspects, andfeatures. It should be readily appreciated that modifications may bemade by persons skilled in the art, and the present applicationcontemplates any and all modifications within the scope and spirit ofthe present disclosure.

1. A process for fabricating a transparent polycrystalline ceramic froma ceramic nanopowder, comprising: coating individual ceramicnanoparticles of the nanopowder with a sintering aid to form a coatednanopowder so as to minimize direct contact of adjacent ceramic powderparticles; baking the coated nanopowder; and subsequently hot pressingthe coated nanopowder to form a solid ceramic.
 2. The process accordingto claim 1, further comprising baking the coated ceramic particles atabout 600° C. for about 4 to about 18 hours.
 3. The process according toclaim 1, further comprising dissolving the sintering aid in a solventand spray coating the dissolved sintering aid on the ceramicnanoparticles.
 4. The process according to claim 1, further comprisingdissolving the sintering aid in a solvent and precipitating thedissolved sintering aid on the surface of the ceramic nanoparticles. 5.The process according to claim 1, wherein the hot pressing step includesapplying pressure to the nanopowder and increasing the temperature ofthe coated nanopowder at a rate of about 20° C./minute to a temperatureof about 1000° C., subsequently increasing the temperature of the coatednanopowder at a rate of about 5° C./minute to a temperature of about1500° C., and holding the heated nanopowder under the pressure at 1500°C. for about 2 hours.
 6. The process according to claim 1, wherein thesintering aid comprises LiF.
 7. The process according to claim 1,wherein the ceramic comprises a Yb³⁺ doped Y₂O₃ having a dopantconcentration of from about 0 to about 20%.
 8. The process according toclaim 1, wherein the ceramic comprises a Yb³⁺ doped Sc₂O₃ having adopant concentration of from about 0 to about 20%.